In the field of electronics manufacturing, it is often necessary to determine the relative alignment or registration between objects or patterns. The registration process typically involves several cooperating functions including the following:
1) Calibration. Calibration defines what is considered correct registration to the process which effects the alignment of two objects.
2) Active Registration Control. The alignment process performs successive adjustments to the relative alignment of two objects by measuring the relative alignment of two fiducial patterns.
3) Process Monitoring: A measurement of mis-registration of two objects with an appropriate sampling plan to detect drift, erratic or out of specification registration.
An exemplary registration process is one involving a machine that prints a pattern upon a printed circuit board. The printing machine requires calibration to ensure that the pattern is printed in the correct location. This calibration takes the form of printing a pattern, measuring the horizontal and vertical offsets of the printed pattern relative to the ideal location, and then adjusting the printing machine to correct for these offsets.
The ability to accurately measure the degree of mis-registration also supports process control and inspection. The registration process is fundamental to several electronics manufacturing applications including but not limited to the following:
1. Alignment of layers of a multi-layer substrate structure such as printed wiring boards, hybrid circuits, flex circuits, multi-chip modules, chip carriers, etc; PA1 2. Alignment of solder paste to corresponding pads; PA1 3. Alignment of successive layers of semiconductor integrated circuit devices; PA1 4. Alignment of solder paste onto substrate; PA1 5. Alignment of solder mask and nomenclature patterns onto printed wiring board; and PA1 6. Alignment of successively printed layers on thick film circuits.
In the process monitoring context, registration detection schemes provide valuable feedback about the accuracy of process steps. For example, the manufacture of electronics assemblies can require that material deposited on the workpiece during a given manufacturing step be precisely aligned with previously-deposited material or with specific patterns on the workpiece. A common example is the alignment of a material referred to as "solder paste" with etch located on a surface of a PC board in preparation for the step of soldering components thereto. During a conventional soldering process, a solder stencil is placed over a component-connection surface of a PC board. The solder stencil has numerous openings corresponding to locations where solder paste is to be placed, such as contact pads that will subsequently receive component leads. After the solder stencil has been placed, solder paste is deposited over it, which results in the paste being "screened" onto the PC board. Then the solder stencil is removed, leaving the PC board with numerous blocks of solder paste on its surface.
Once the solder paste has been screened onto the board, the components are then placed on the board. Ideally, the solder paste blocks are all perfectly aligned with the component connection pads, and each component lead is slightly embedded in its corresponding solder paste block. However, inaccuracies in the solder paste screening process may cause the solder paste blocks to be misaligned with the pads. If this misalignment is beyond an acceptable limit, the likelihood that the assembly process results in defective or low-quality circuits increases, resulting in corresponding increases in test and repair costs. Therefore, monitoring and control of the solder paste deposition step is important to achieving good electronic assembly yields. Ideally, of course, such monitoring should be done as quickly and accurately as possible, so that effective monitoring is obtained at a minimum cost.